Mr. Broder's reply in the other thread was "I cannot account for the discrepancy, nor for a later stretch in Connecticut where I recall driving about 45 m.p.h., but it may be the result of the car being delivered with 19-inch wheels and all-season tires, not the specified 21-inch wheels and summer tires."

I dont know how it measures speed whether GPS or wheel rotation or both and if that would make a difference. Anyone care to fact check?

Looking at the Tesla Model S website[0] both 19" and 21" wheels (not tires) are available. The 19" wheels have 245/45R19 tires and the 21" wheels have 245/35R21. Punching that in to a tire size calculator[1] shows the following:

Tire Size Comparison

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference

245/45-19 4.3in 13.8in 27.7in 87.0in 729 0.0%

245/35-21 3.4in 13.9in 27.8in 87.2in 727 0.3%

Which is a 0.3% difference in total circumference. Which is not a significant source of discrepency.

[0] - http://www.teslamotors.com/models/specs [1] - http://www.miata.net/garage/tirecalc.html (I used the non-java form at the bottom of the page)

The 19 / 21 inch is not the outside diameter of the tires, it's the inside diameter. Outside diameter would be about the same and would not make much of a difference.